383 
ON DIFFERENTIAL EARTH-MOVEMENT IN NORTH-EAST 
YORKSHIRE DURING THE JURASSIC PERIOD. 
By G. W. Lamplugh, F.R.S. 
Presidential Address. 
(Read at York, November 16th, 1921.) 
In dealing with the structure of the Weald three years ago, in a 
Presidential Address to the Geological Society of London, I referred 
briefly to the wedge-like arrangement of the Jurassic rocks of York- 
shire, and mentioned that these deposits, like those of the south- 
country Weald, yielded e\idence that they similarly were accumulated 
in a slowly sinking trough.* On the present occasion I propose to 
enter into some of the particulars relating to this matter, and to apply 
to the Yorkshire Oolites the method adopted for the Wealden rocks.f 
There is nothing novel in the method. It was applied by Mr. 
Cosmo Johns to the Coal Measures of Yorkshire in a suggestive paper 
published by our Society in 1906 ;{ but, so far as I am aware, it has 
not previously been tested in respect to the Yorkshire Jurassic sequence. 
It is simple in essence, and may be stated as follows : — 
When a particular bed in a stratified sequence bears evidence 
of having been deposited at the same depth, and at about the same 
time, over a wide area, it marks a plane originally parallel to the sea- 
level of its period — i.e., a plane of horizontality. 
When several beds of this kind occur at intervals in the sequence 
and are not parallel to each other, we must infer that there has been 
relative change in the local plane of horizontality during the accumu- 
lation of the sequence, and we are enabled to determine the 
direction and amount of the change. 
Glacial and Post-Glacial marine beaches have long been recognized 
as affording data of this kind ; and the beaches of older periods would 
be equally useful if we could be sure of their s}Tichronism throughout 
their extent, which is often doubtful. Estuarine or brack-water 
* Quart. Journ. Geol. Soc, Vol. LXXV. (for 1919), pp. Ixxxvii-ix. 
t "On the Mesozoic Rocks in some of the Coal Explorations in Kent,"' 
by G. W. Lamplugh and F. L. Kitchin. Mem. Geol. Surv., 1911, 
pp. 88-94. 
X "On Differential Earth Movements during Carboniferous Times, 
etc." Proc, YorJcs. Geol. Soc. for 1905. N.S., Vol. XV., pt. iii., 
pp. 372-9. 
2c 
